In an electronic controlled engine idling speed control system of an internal combustion engine for an automotive vehicle which has become increasingly popular, an intake air control system is used together with an electronic controlled fuel injection system. This type of intake air control system has an intake air passage bypassing a throttle valve disposed in an intake manifold and an intake air control means, such as an electronic controlled throttle valve, provided in the bypass passage for controlling the amount of intake air flowing through the bypass passage when the throttle valve is in the idle position, or at the minimum throttle opening. The amount of intake air is feedback controlled in accordance with the difference between an actual engine speed detected by means of an RPM sensor, or a crank angle sensor, and a desired engine idling speed so as to operate the internal combustion engine at the desired engine idling speed.
To perform this feedback control, an engine control unit (ECU), which generally consists of a microprocessor, is used as an intake air feedback control means. The engine control unit feedback controls the intake air control means by the use of primary intake air flow controlling values previously provided in accordance with desired engine idling speeds. When the actual engine speed of rotation of the internal combustion engine thus controlled by the use of the primary intake air flow controlling value disagrees with the desired engine idling speed, the primary intake air flow controlling value is, either increasingly or decreasingly, correctively changed in accordance with the difference between the actual engine speed and the desired engine idling speed, and loadings on the internal combustion engine, so as to make the actual engine speed agree with the desired engine idling speed.
An eventual controlling value G for controlling the intake air control means, in which the difference in speed and loads on the internal combustion engine are taken into account, is expressed in the following general formula: EQU G=G.sub.B +.SIGMA.G.sub.L +G.sub.FB (I)
wherein G.sub.B is the primary intake air controlling value,
.SIGMA.G.sub.L is the load correcting value depending on loadings on the internal combustion engine, and PA1 G.sub.FB is the feedback correcting value in accordance with the difference between the actual engine speed and the desired engine idling speed.
The primary intake air controlling value G.sub.B generally depends upon the temperatures of cooling water based on the inherent primary value of the internal combustion engine running with no loading. The load-depending correcting value .SIGMA.G.sub.L is determined corresponding to the sum of values inherent in various kinds of loadings, such as an air conditioning system, a power steering system, a headlight system, etc. This load-depending correcting value serves to cause an uniform increase of intake air flow. The speed-depending feedback correcting value G.sub.FB is a value used in closed-loop control which is determined depending upon the difference between the actual engine speed and the desired engine idling speed and upon the change in engine operating conditions.
As apparent from the above general formula (I) from which is calculated a duty ratio of a control signal at which a solenoid means of the intake air control means should be operated, the eventual controlling value G is determined based mainly on a particular fixed value inherent in the internal combustion engine and the temperature of cooling water, which is a parameter representative of a state of the internal combustion engine, and subordinately on the load-depending correcting value .SIGMA.G.sub.L and the speed-depending feedback correcting value G.sub.FB depending upon the difference .DELTA.N.sub.E between the actual engine speed N.sub.E and the desired engine idling speed N.sub.OID which depends upon the temperature of cooling water.
Control of the amount of intake air by the use of the speed-depending feedback correcting value G.sub.FB is effected only in a feedback control range, hereinafter referred simply to as an F/B range, while the internal combustion engine idles. Judgement of the F/B range is made in accordance with the conditions of an idle switch (which is turned on when the throttle valve is in its idle position) and a primary engine speed N.sub.OFB (which is determined inherently in the characteristics of the internal combustion engine and somewhat higher than the desired engine idling speed N.sub.OID). That is, the engine operating condition is judged to be in the F/B range when the condition that the idle switch is turned on and the actual engine speed N.sub.E drops down lower than the primary engine speed N.sub.OFB is satisfied.
In an application of the engine idling speed control system described above to an automotive vehicle equipped with an automatic transmission of the type having a torque converter (which is hereinafter referred to simply as an A/T vehicle), a problem of creeping is encountered. That is, if the throttle valve is completely closed, or in its idle position, by removing one's foot from an acceleration pedal when the brake is completely released, although the internal combustion engine is idling when the automatic transmission is shifted from a non-driving range, such as a park range (P) or a neutral range (N), into a driving range, such as a drive range (D), the A/T vehicle generally starts and creeps at a low speed of about 3 to 4 Km/h. As is apparent, in automatic transmissions of the type described above, the automatic transmission fluid-couples the output shaft of the internal combustion engine to the driven shaft of the torque converter in the driving range, such as the drive range (D), unlike in the non-driving range, such as the neutral range (N). Thus the engine output torque, depending upon the number of rotations of the idling engine, is transmitted to a drive line of the A/T vehicle, so that, on one hand, the A/T vehicle is driven or creeps and, on the other hand, this creeping of the A/T vehicle puts loads on the idling engine. As a result, the engine speed N.sub.E unavoidably becomes lower by a certain number of revolutions per second than an engine idling speed in the neutral range (N) or in the park range (P) when the automatic transmission shifts itself to the driving range from the non-driving range.
To prevent such a drop of engine speed, the amount of intake air is increasingly controlled depending on loadings on the internal combustion engine from the automatic transmission. For this control, a load-depending correcting value G.sub.L inherent in the automatic transmission itself is previously given so as to automatically increase the amount of intake air, thereby preventing a drop of engine speed. Such a feedforward control system is known from Japanese Unexamined Patent Publication No. 60-19,933 entitled "Method of Controlling Rotational Speed of Internal Combustion Engine", published Feb. 1, 1985.
Loadings on the internal combustion engine from the automatic transmission in the driving range, such as the drive range (D), are significantly different between when either a foot-brake or a hand-brake, or both, are applied to stop the A/T vehicle and when these brakes are released, thereby allowing the A/T vehicle to creep. That is, while the A/T vehicle is creeping, because the relative turbine and pump speeds of the torque converter drops, the torque converter encounters an increased frictional drag of a coupling fluid and the A/T vehicle produces a running inertia, resulting in an increase of stability in rotation of the internal combustion engine. Accordingly, loadings on the internal combustion engine from the automatic transmission are considerably lowered in creeping conditions relative to in braked conditions which the pump of the torque converter continues to rotate while the turbine of the torque converter is locked.
Reference should now made be to FIG. 1(A)-(C) to illustrate an example wherein the A/T vehicle, stationarily running at a certain speed V (in Km/h) with the internal combustion engine operating at a speed rotation of N.sub.E (in rpm) begins to decelerate at a time t.sub.1. When the throttle valve is fully closed at the time t.sub.1 to decelerate the automotive vehicle without application of brake force, the A/T vehicle gradually loses speed as the speed of the internal combustion engine drops. It is, however, to be noted that there is a slight time delay between the decrease of the vehicle speed and of the engine speed. At a time t.sub.2, the internal combustion engine is slowed to the primary engine speed N.sub.OFB (which is determined to be approximately 600 rpm greater than the desired engine idling speed N.sub.OID in order to avoid down shooting), and feedback control is initiated to control the amount of intake air into the internal combustion engine so that the internal combustion engine drifts down to the desired engine idling speed N.sub.OID.
At this time, however, because the actual engine speed N.sub.E is still greater than the engine idling speed N.sub.OID and the running speed V of the A/T vehicle is significantly greater than a speed at which the vehicle creeps due to inertial running, a loading on the internal combustion engine from the automatic transmission is quite considerably small. Accordingly, the load-depending correcting value G.sub.L depending upon the transmission loading is primarily small and, before the drifting of the engine speed N.sub.E down to the desired engine idling speed N.sub.OID, the speed-depending feedback controlling value G.sub.FB in intake air F/B control is decreasingly changed to a limit value or guard value G.sub.FB(MIN) as shown in FIG. 1(C), so that the A/T vehicle moves into creeping with an absolute amount of intake air left decreased. This leads to an enormous down shoot of engine speed, which causes the internal combustion engine to stall upon suddenly braking the A/T vehicle while creeping, not only because of the sluggishness of correctively responding to a rapidly increased loading on the internal combustion engine from the automatic transmission that requires a great amount of intake air but also because the engine speed N.sub.E has dropped down to the desired engine idling slow N.sub.OID.